Force limiting devices are known in general from various applications in motor vehicles. Steering wheels are known, for example, which collapse in case of a crash due to the impact with the seat occupant, and have a device for energy absorption, by means of which a force acting on the vehicle occupant is limited. Flexible deformation elements are furthermore provided in the entire vehicle body structure, so that energy is absorbed in the event of a crash, so that the force acting on the passenger compartment is limited.
It is furthermore known from the prior art to provide force limiting elements in the safety belts, so that the force acting on passengers in a crash due to forward displacement is limited by the force limiting device. Two-part belt shafts with a torsion rod arranged between the two parts of the belt shaft have proven useful in the practice. In case of a crash, one part of the belt shaft is locked with respect to the vehicle, while the other part is attached to the safety belt and rotates in the belt extension direction. During the relative rotation of the two parts to each other, the torsion rod arranged between the two parts is plastically deformed about its own axis in an energy consuming manner, so that the force in the safety belt during the forward displacement of the passenger is limited accordingly. The amount of force limiting is determined by the torsion rod and cannot be varied.
An innovative, frequency-controlled force limiting system is known from patent publication WO 2006/108451 A1 of the applicant. The force limiting system described therein consists of two parts moving with respect to each other, whose movement with respect to each other is controlled by a mass system caused to vibrate or oscillate at a defined frequency. The nullification of the energy takes place in that the parts are alternately decelerated and accelerated, wherein the frequency is essentially independent of the applied force. Different force limiting levels can therefore be attained with this force limiting system as a function of the mass of the moving parts and of the momentum occurring in case of a vehicle impact, wherein the frequency and the path of the forward displacement are approximately constant.
From WO 2010/037460 A2 is known further development of the force limiting system described therein, in which the force-limited forward displacement of the passenger is made possible in that a part is provided which engages in a gearing of another part and performs a wave-like forward movement in which the gearing is alternately engaged and disengaged.
The characteristic of the force limiting curve in a system of this kind can be changed by the geometry of the gearing and by the mass of the moving parts. The force limiting characteristic basically can no longer be changed after the part having the gearing has been produced.